This invention relates to a method and apparatus for making a metal annular member of precise tolerances and also to a method for providing desired surface finish.
Suspension systems, particularly those suspension systems found in automotive applications, make substantial use of devices commonly known as bushings. Generally these bushings consist of at least two more or less concentric annular members, usually of metal, separated by a rubber element of specific design which element is under compression. Such devices are sometimes called outer and inner metals or "outers" and "inners" and are more fully described in, for example, U.S. Pat. Nos. 3,893,775; 3,495,859; and 3,199,186. The present procedure for making these annular members involves several draw die steps wherein an initially flat blank is drawn in successive draw die procedures into the desired configuration. This procedure is not entirely satisfactory because it is expensive, requiring repeated transfer steps, and it produces a product not always as accurate as desired. The relevant prior art may also include the U.S. Pat. Nos. 2,506,657 to Webster; 3,224,243 to VanDerberg; 3,789,650 to Axeloff; 3,263,477 to Roper; 2,930,483 to Kaul; 3,314,278 to Bergman; 3,668,918 to Benteler; and 3,740,993 to Moore.
In certain bushings it is desirable or necessary that the surface of the outer or inner which contacts the rubber element be so configured or finished as to grip the rubber element so as to eliminate or reduce slippage therebetween. One prior art procedure for preventing relative movement between the metal sleeves and the elastomeric insert is disclosed in Sievers U.S. Pat. No. 3,893,775 and involves sandblasting the surfaces of the metal sleeves and thereafter forming a phosphate coating thereon. It is desirable that improved means be provided for forming a rough gripping surface on the metal sleeves of the bushing. It is desirable that better control over the precise form of the surface texture be provided and that this be accomplished inexpensively. Other prior art related to this feature are U.S. Pat. Nos. 1,940,302 to Humphre; 1,959,256 to Zerk; 2,725,692 to Andreae; 2,819,105 to Belinke; 3,368,852 to Herbenar; and 3,504,513 to Black.